Digital data are conventionally stored on optical storage media as binary data in the form of information elements, e.g. as pits. These elements are generally distributed over one or more plane surfaces within the medium, called information layers. In the case of a conventional optical disk, e.g. a Compact Disk (CD), Digital Versatile Disk (DVD), or BluRay Disk (BD), the information elements of the same layer are distributed in a spiral track or concentric circular tracks.
The maximum storage density on an information layer is limited by the minimum size of the information elements and by the minimum distance between adjacent tracks. In practice, in order to be able to read the stored data by conventional optical detection means, this minimum size and distance are determined by the wavelength of the light used for the optical detection.
To overcome the limitation of the storage density, a spectrally coded data storage has been proposed. For example, JP05-062239 discloses a storage medium for multiple wavelength storage. The storage medium has a layer of an amorphous matrix in which nanoparticles with different sizes are distributed. The different sizes lead to different resonance frequencies.
The article entitled “Spectrally coded optical data storage by metal nanoparticles” by H. Diltbacher et al., Review Optics Letters, Vol. 25, No. 8, 1995, pages 563-565, indicates that the use of non-linear optical technologies affords solutions to increase the storage density of a storage layer. The authors stipulate that, if the spectral composition of the light scattered by an information element can be made dependent on a parameter relating to the information element, such as the shape, the amount of information carried by an information element and as a result the optical storage density would be increased. To this end, the document teaches arranging, on a medium or surface that is used for storage, metal particles of sizes less than that of the wavelength of the light, and of different size and/or shape and/or orientation.
Thus, when this surface is suitably illuminated, resonant modes of groups of electrons, called “localized plasmons”, are excited within the metal nanoparticles, which causes absorption of certain wavelengths of the incident radiation. Since the excitation of these resonant modes depends on the shape, the orientation and the distribution of the nanoparticles, this leads to spectrally coded or “polychromatic” data storage. Using the “polychromatic” storage method, the maximum storage density is noticeably increased by a factor of about five compared to the conventional “monochrome” optical storage modes.
More specifically, according to the article silver nanoparticles are deposited on a transparent substrate using a cathodo-lithographic process. In order to optically read data stored in this coating, a localized electron plasma is excited within the nanoparticles by means of an evanescent electromagnetic field obtained by total internal reflection of radiation incident on the surface of the substrate. For calculating the amplitude of the plasmon resonance, the intensity of the light scattered by this surface is measured as a function of the wavelength using conventional optical detection means.